Tubeless tires have in fact an inner surface of low air permeability so as to prevent the tire from deflating and to protect the sensitive internal regions of said tire from the ingress of oxygen and water, such as the plies containing oxidation-sensitive metal cords, this protection improving the endurance of the tire. At the present time, such protection of the inner surface of tires is achieved by inner liners consisting of elastomeric compositions based on butyl rubber.
However, since fuel savings and the need to protect the environment have become priorities, it is desirable to produce air-impermeable inner liners with the lowest possible weight and smallest possible hysteresis so as to improve the rolling resistance of the tire. The air impermeability performance of butyl rubbers is dependent on having a not insignificant minimum thickness (of the order of one millimeter) and therefore a certain weight, which means that these new requirements are not effectively met.
Thus, it is necessary to add reinforcing fillers, such as carbon black, to the elastomeric composition of the inner liner in order to improve its impermeability. However, in large quantities these reinforcing fillers impair certain properties of the composition both in the green state—difficulty in processing the green composition, i.e. processability difficulty—and in the cured state—degradation in mechanical properties, especially a reduction in flexural strength. By introducing plasticizers of the oil type it is possible to alleviate these processing and mechanical property aspects, but very greatly to the detriment of impermeability.
Various solutions have been envisaged for remedying these drawbacks, in particular by the use of other types of fillers, often known as smectites and in particular as organophilic smectites, to be added to the reinforcing fillers. These organophilic smectites improve the impermeability properties of a material if they are well dispersed in the material, i.e. if these fillers are both uniformly distributed within the material and very compatible with the latter. This dispersion is often difficult to achieve owing to the poor thermodynamic compatibility that exists between the elastomers and such fillers.
The publication WO 2006/047509 by the Applicant describes a tire inner liner composition based on butyl rubber and containing carbon black, includes non-reinforcing fillers consisting of organophilic smectites that reduce the gas permeability, said fillers being dispersed in the elastomeric matrix, and also a specific plasticizer consisting of a terpene resin having in particular a glass transition temperature Tg above 50° C. This composition does actually have mechanical and impermeability properties that make it acceptable for use as a tire inner liner thanks to the combined effect of these organophilic smectites and this high-Tg resin.
The Applicant has continued its research and has surprisingly discovered a rubber composition for a tire inner liner based on at least one butyl rubber, a reinforcing filler, graphite and a hydrocarbon plasticizing resin, which composition has processability and flexural strength properties as good as the compositions of the prior art, and improved gas impermeability properties, and which also makes it possible for the rolling resistance and endurance properties to be significantly improved without, of course, this being to the detriment of other properties.
In addition, the Applicant has found, surprisingly, that hydrocarbon plasticizing resins having a Tg above 20° C., and preferably above 30° C., make it possible to obtain, in combination with graphite, properties as good as for the more restrictive combination of resins having a Tg above 50° C. with organophilic smectites.